The present invention relates to a ribbon-type speaker. More specifically, the invention relates to the structure of a ribbon-type speaker in which a conductive ribbon disposed in a d-c magnetic field produced in the gap of a magnetic circuit is used as a vibrating plate and to a method of assembling to magnetic circuit unit thereof.
Generally, in a ribbon-type speaker, sound waves are generated by passing an audio signal current through a conductive ribbon and by thus vibrating the ribbon in response to the amplitude and the direction of the current. Essential components of the ribbon-type speaker are a magnetic circuit and a vibration system.
A conventional ribbon-type speaker includes, as shown in FIG. 1, a pair of permanent magnets 1a and 1b, a yoke 2 for magnetically coupling the permanent magnets 1a and 1b, and a pair of magnetic pole pieces 9a and 9b mounted adjacent respective ends of the permanent magnets 1a and 1b in a magnetic circuit. A gap in the magnetic circuit is formed between the pole pieces 9a and 9b. A conductive ribbon 3, which is supported at both ends by a supporting member 4 mounted on the pole pieces 9a and 9b, is disposed in the gap of the magnetic circuit. An audio signal current flows from a matching transformer 14 through the ribbon 3 coupled through a band-like feeder 16. The ribbon 3 and the feeder 16 and the related components form a vibration system in the speaker.
In the conventional ribbon-type speaker as described above, the components in the magnetic circuit are integrally assembled with the components in the vibration system. More particularly, the speaker is constructed by assembling the components of the vibrations system in the magnetic circuit. accordingly, such a conventional speaker has a drawback that the light ribbon and the heavy magnetic circuit must be simultaneously handled thus lowering the assembling efficiency.
Also if the permanent magnets of the magnetic circuit are magnetized prior to assembly, their assembly is very difficult.
Another conventional ribbon-type speaker, as shown in FIG. 2, includes a magnet 1, a pair of yokes 2 forming a magnetic circuit together with the magnet 1 with a magnetic air gap G formed between the yokes 2, a ribbon-like vibrating plate 3 arranged in the air gap G, and damper members 6 made of viscoelastic material interposed between the sides of the vibrating plate 3 and the yokes 2.
When assembling such a conventional speaker, specifically, when assembling the vibration system with the magnetic circuit, the vibrating plate 3 is set in the magnetic air gap G of the previously assembled magnetic circuit and the damper member 6 is positioned in the gap between the vibrating plate 3 and the yokes 2. Thus, the extremely thin and deformable vibrating plate 3 must be handled directly which must be done very carefully. This results in a reduction in productivity and an increased cost.
The applicants have previously proposed an improved ribbon-type speaker, as shown in FIGS. 3 to 5, in which it is not necessary to directly handle a vibrating plate 3 when assembling the vibrating plate 3 with the magnetic circuit. The assembly in this case in accomplished by superimposing, as illustrated in FIG. 3, a supporting frame 5 made of aluminum plate or paper or the like in which a rectangular opening 5' is formed having a long side shorter than the length of the ribbon-like vibrating plate 3 and a short side longer than the width of the vibrating plate 3, and a damper member 6 made of viscoelastic film in which an opening 6' is formed having a long side equal in length to the length of the opening 5' and a shorth side shorter than the width of the vibrating plate 3 with the center of the two openings 5' and 6' coinciding with each other. The periphery of the damper member 6 is fusion bonded or adhered with an adhesive to the supporting frame 5. Thereafter, the vibrating plate 3 is placed upon on the damper member 6 so as to block the opening 6' of the damper member 6. The periphery of the opening 6' of the damper member 6 is fusion bonded or adhered with an adhesive to the vibrating plate 3 to thus complete the vibration system assembly. As shown in FIG. 4, the vibrating system assembly is mounted to the yokes 2 to thus arrange the vibrating plate 3 in the air gap G of the magnetic circuit. In FIG. 4, reference numeral 7 indicates sub yokes, which respectively clamp the periphery of the supporting frame 5 through a packing 8 with the stepped portions 2' of the main yokes 2 and secure the vibration system assembly to the magnetic circuit.
With the vibrating plate 3 supported through the film-like damper member 6 by the supporting frame 5 as described above, when the vibrating plate 3 is vibrated, the supporting frame 5 is damped at the corners 5", as shown in FIG. 5, since the damper member 6 extends higher than the other portions. Accordingly, stress is applied to the vibrating plate 3 resulting in deformation of the vibrating plate 3. Consequently, this stress produces a variation in the stiffness of the vibrating plate 3 in the vibrating direction and thus deteriorates the quality of reproduced sound due to the occurrence of second harmonic wave strain. Further, this stress tends to cause peeling of the vibrating plate 3 from the damper member 6.